linux/Documentation/vm/slub.txt
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   1Short users guide for SLUB
   2--------------------------
   3
   4The basic philosophy of SLUB is very different from SLAB. SLAB
   5requires rebuilding the kernel to activate debug options for all
   6slab caches. SLUB always includes full debugging but it is off by default.
   7SLUB can enable debugging only for selected slabs in order to avoid
   8an impact on overall system performance which may make a bug more
   9difficult to find.
  10
  11In order to switch debugging on one can add a option "slub_debug"
  12to the kernel command line. That will enable full debugging for
  13all slabs.
  14
  15Typically one would then use the "slabinfo" command to get statistical
  16data and perform operation on the slabs. By default slabinfo only lists
  17slabs that have data in them. See "slabinfo -h" for more options when
  18running the command. slabinfo can be compiled with
  19
  20gcc -o slabinfo tools/vm/slabinfo.c
  21
  22Some of the modes of operation of slabinfo require that slub debugging
  23be enabled on the command line. F.e. no tracking information will be
  24available without debugging on and validation can only partially
  25be performed if debugging was not switched on.
  26
  27Some more sophisticated uses of slub_debug:
  28-------------------------------------------
  29
  30Parameters may be given to slub_debug. If none is specified then full
  31debugging is enabled. Format:
  32
  33slub_debug=<Debug-Options>       Enable options for all slabs
  34slub_debug=<Debug-Options>,<slab name>
  35                                Enable options only for select slabs
  36
  37Possible debug options are
  38        F               Sanity checks on (enables SLAB_DEBUG_FREE. Sorry
  39                        SLAB legacy issues)
  40        Z               Red zoning
  41        P               Poisoning (object and padding)
  42        U               User tracking (free and alloc)
  43        T               Trace (please only use on single slabs)
  44        A               Toggle failslab filter mark for the cache
  45        O               Switch debugging off for caches that would have
  46                        caused higher minimum slab orders
  47        -               Switch all debugging off (useful if the kernel is
  48                        configured with CONFIG_SLUB_DEBUG_ON)
  49
  50F.e. in order to boot just with sanity checks and red zoning one would specify:
  51
  52        slub_debug=FZ
  53
  54Trying to find an issue in the dentry cache? Try
  55
  56        slub_debug=,dentry
  57
  58to only enable debugging on the dentry cache.
  59
  60Red zoning and tracking may realign the slab.  We can just apply sanity checks
  61to the dentry cache with
  62
  63        slub_debug=F,dentry
  64
  65Debugging options may require the minimum possible slab order to increase as
  66a result of storing the metadata (for example, caches with PAGE_SIZE object
  67sizes).  This has a higher liklihood of resulting in slab allocation errors
  68in low memory situations or if there's high fragmentation of memory.  To
  69switch off debugging for such caches by default, use
  70
  71        slub_debug=O
  72
  73In case you forgot to enable debugging on the kernel command line: It is
  74possible to enable debugging manually when the kernel is up. Look at the
  75contents of:
  76
  77/sys/kernel/slab/<slab name>/
  78
  79Look at the writable files. Writing 1 to them will enable the
  80corresponding debug option. All options can be set on a slab that does
  81not contain objects. If the slab already contains objects then sanity checks
  82and tracing may only be enabled. The other options may cause the realignment
  83of objects.
  84
  85Careful with tracing: It may spew out lots of information and never stop if
  86used on the wrong slab.
  87
  88Slab merging
  89------------
  90
  91If no debug options are specified then SLUB may merge similar slabs together
  92in order to reduce overhead and increase cache hotness of objects.
  93slabinfo -a displays which slabs were merged together.
  94
  95Slab validation
  96---------------
  97
  98SLUB can validate all object if the kernel was booted with slub_debug. In
  99order to do so you must have the slabinfo tool. Then you can do
 100
 101slabinfo -v
 102
 103which will test all objects. Output will be generated to the syslog.
 104
 105This also works in a more limited way if boot was without slab debug.
 106In that case slabinfo -v simply tests all reachable objects. Usually
 107these are in the cpu slabs and the partial slabs. Full slabs are not
 108tracked by SLUB in a non debug situation.
 109
 110Getting more performance
 111------------------------
 112
 113To some degree SLUB's performance is limited by the need to take the
 114list_lock once in a while to deal with partial slabs. That overhead is
 115governed by the order of the allocation for each slab. The allocations
 116can be influenced by kernel parameters:
 117
 118slub_min_objects=x              (default 4)
 119slub_min_order=x                (default 0)
 120slub_max_order=x                (default 3 (PAGE_ALLOC_COSTLY_ORDER))
 121
 122slub_min_objects allows to specify how many objects must at least fit
 123into one slab in order for the allocation order to be acceptable.
 124In general slub will be able to perform this number of allocations
 125on a slab without consulting centralized resources (list_lock) where
 126contention may occur.
 127
 128slub_min_order specifies a minim order of slabs. A similar effect like
 129slub_min_objects.
 130
 131slub_max_order specified the order at which slub_min_objects should no
 132longer be checked. This is useful to avoid SLUB trying to generate
 133super large order pages to fit slub_min_objects of a slab cache with
 134large object sizes into one high order page. Setting command line
 135parameter debug_guardpage_minorder=N (N > 0), forces setting
 136slub_max_order to 0, what cause minimum possible order of slabs
 137allocation.
 138
 139SLUB Debug output
 140-----------------
 141
 142Here is a sample of slub debug output:
 143
 144====================================================================
 145BUG kmalloc-8: Redzone overwritten
 146--------------------------------------------------------------------
 147
 148INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
 149INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
 150INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
 151INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
 152
 153Bytes b4 0xc90f6d10:  00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
 154  Object 0xc90f6d20:  31 30 31 39 2e 30 30 35                         1019.005
 155 Redzone 0xc90f6d28:  00 cc cc cc                                     .
 156 Padding 0xc90f6d50:  5a 5a 5a 5a 5a 5a 5a 5a                         ZZZZZZZZ
 157
 158  [<c010523d>] dump_trace+0x63/0x1eb
 159  [<c01053df>] show_trace_log_lvl+0x1a/0x2f
 160  [<c010601d>] show_trace+0x12/0x14
 161  [<c0106035>] dump_stack+0x16/0x18
 162  [<c017e0fa>] object_err+0x143/0x14b
 163  [<c017e2cc>] check_object+0x66/0x234
 164  [<c017eb43>] __slab_free+0x239/0x384
 165  [<c017f446>] kfree+0xa6/0xc6
 166  [<c02e2335>] get_modalias+0xb9/0xf5
 167  [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
 168  [<c027866a>] dev_uevent+0x1ad/0x1da
 169  [<c0205024>] kobject_uevent_env+0x20a/0x45b
 170  [<c020527f>] kobject_uevent+0xa/0xf
 171  [<c02779f1>] store_uevent+0x4f/0x58
 172  [<c027758e>] dev_attr_store+0x29/0x2f
 173  [<c01bec4f>] sysfs_write_file+0x16e/0x19c
 174  [<c0183ba7>] vfs_write+0xd1/0x15a
 175  [<c01841d7>] sys_write+0x3d/0x72
 176  [<c0104112>] sysenter_past_esp+0x5f/0x99
 177  [<b7f7b410>] 0xb7f7b410
 178  =======================
 179
 180FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
 181
 182If SLUB encounters a corrupted object (full detection requires the kernel
 183to be booted with slub_debug) then the following output will be dumped
 184into the syslog:
 185
 1861. Description of the problem encountered
 187
 188This will be a message in the system log starting with
 189
 190===============================================
 191BUG <slab cache affected>: <What went wrong>
 192-----------------------------------------------
 193
 194INFO: <corruption start>-<corruption_end> <more info>
 195INFO: Slab <address> <slab information>
 196INFO: Object <address> <object information>
 197INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
 198        cpu> pid=<pid of the process>
 199INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
 200         pid=<pid of the process>
 201
 202(Object allocation / free information is only available if SLAB_STORE_USER is
 203set for the slab. slub_debug sets that option)
 204
 2052. The object contents if an object was involved.
 206
 207Various types of lines can follow the BUG SLUB line:
 208
 209Bytes b4 <address> : <bytes>
 210        Shows a few bytes before the object where the problem was detected.
 211        Can be useful if the corruption does not stop with the start of the
 212        object.
 213
 214Object <address> : <bytes>
 215        The bytes of the object. If the object is inactive then the bytes
 216        typically contain poison values. Any non-poison value shows a
 217        corruption by a write after free.
 218
 219Redzone <address> : <bytes>
 220        The Redzone following the object. The Redzone is used to detect
 221        writes after the object. All bytes should always have the same
 222        value. If there is any deviation then it is due to a write after
 223        the object boundary.
 224
 225        (Redzone information is only available if SLAB_RED_ZONE is set.
 226        slub_debug sets that option)
 227
 228Padding <address> : <bytes>
 229        Unused data to fill up the space in order to get the next object
 230        properly aligned. In the debug case we make sure that there are
 231        at least 4 bytes of padding. This allows the detection of writes
 232        before the object.
 233
 2343. A stackdump
 235
 236The stackdump describes the location where the error was detected. The cause
 237of the corruption is may be more likely found by looking at the function that
 238allocated or freed the object.
 239
 2404. Report on how the problem was dealt with in order to ensure the continued
 241operation of the system.
 242
 243These are messages in the system log beginning with
 244
 245FIX <slab cache affected>: <corrective action taken>
 246
 247In the above sample SLUB found that the Redzone of an active object has
 248been overwritten. Here a string of 8 characters was written into a slab that
 249has the length of 8 characters. However, a 8 character string needs a
 250terminating 0. That zero has overwritten the first byte of the Redzone field.
 251After reporting the details of the issue encountered the FIX SLUB message
 252tells us that SLUB has restored the Redzone to its proper value and then
 253system operations continue.
 254
 255Emergency operations:
 256---------------------
 257
 258Minimal debugging (sanity checks alone) can be enabled by booting with
 259
 260        slub_debug=F
 261
 262This will be generally be enough to enable the resiliency features of slub
 263which will keep the system running even if a bad kernel component will
 264keep corrupting objects. This may be important for production systems.
 265Performance will be impacted by the sanity checks and there will be a
 266continual stream of error messages to the syslog but no additional memory
 267will be used (unlike full debugging).
 268
 269No guarantees. The kernel component still needs to be fixed. Performance
 270may be optimized further by locating the slab that experiences corruption
 271and enabling debugging only for that cache
 272
 273I.e.
 274
 275        slub_debug=F,dentry
 276
 277If the corruption occurs by writing after the end of the object then it
 278may be advisable to enable a Redzone to avoid corrupting the beginning
 279of other objects.
 280
 281        slub_debug=FZ,dentry
 282
 283Christoph Lameter, May 30, 2007
 284
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